Bismillah
as salam alykom Umm
The law is already clear and obvious. It had nothing to do with the level of understanding but rather accepting.
As for John understanding or not, he does understand. He never stated other wise. He disapproves though. No matter how u explain, it is just the acceptance. If u noticed his development, he is growing aggressive which reflects a more seeing the truth. When one is on a wrong track and sees the truth, he/she usually grows aggressive towards those who present or let us say represent the truth. He/she is fighting his battle to continue deceving him/herself.
May be I am brief but I think clear enough.